1. Technical Field
The invention is related to a method of operating a high density plasma CVD reactor having inductive and capacitive coupling modes which can be selected individually or in combination for chamber cleaning operations.
2. Background Art
Plasma reactors used for semiconductor processes such as chemical vapor deposition (CVD), etching, reactive ion etching and so forth typically employ either inductive coupling or capacitive coupling to strike and maintain a plasma. Typically, an etch reactor employs capacitive coupling because the high ion energies characteristic of capacitively coupled plasmas are suitable for removing films such as, for example, silicon dioxide. The high ion energies arise from the high electric fields required to strike a capacitively coupled plasma. For example, as disclosed in European Patent Application publication No. 0 552 491 A1 by Collins et al. entitled xe2x80x9cPlasma Etch Processxe2x80x9d published Jul. 28, 1993, a capacitively coupled reactor has parallel electrodes, including a pedestal.
However, the plasma density of a capacitively coupled plasma is not sufficient for performing simultaneous chemical vapor deposition (CVD) and in-situ sputtering at low pressures. Accordingly, a CVD plasma reactor of the type disclosed in U.S. patent application Ser. No. 08/113,776 entitled xe2x80x9cHigh Density Plasma CVD and Etching Reactorxe2x80x9d filed Aug. 27, 1993 by Fairbairn and Nowak employs inductive coupling to strike and maintain the plasma. In the Fairbairn and Nowak application, a domed-shaped helical coil in a dome-shaped chamber ceiling performs the inductive coupling to the plasma at a low chamber pressure (below 100 milliTorr), the dome shape providing a more uniform ion density distribution across the semiconductor wafer. Such an inductively coupled plasma is characterized by a high ion density and is therefore suitable for low pressure CVD plasma processing at high processing (e.g., deposition) rates.
The main differences between capacitively coupled plasmas and inductively coupled plasmas are the following: An inductively coupled plasma has a plasma density which is up to two orders of magnitude higher, thereby providing higher processing rates. A capacitively coupled plasma has a higher electrode sheet voltage, higher self-biasing and higher plasma impedance, and therefore provides higher sputtering rates and greater risk of damage to the wafer. An inductively coupled plasma exhibits lower ion energy distribution, providing a lower risk of damage to the wafer.
A requirement of all plasma reactors, and especially plasma CVD reactors, is that they must be cleaned periodically to remove deposits and residue. For example, in a CVD reactor, during deposition of silicon dioxide onto a semiconductor wafer, silicon dioxide is deposited onto the chamber walls and ceiling as well as other parts of the reactor chamber, changing the characteristics of the chamber and giving rise to particulate contamination. Therefore, the reactor chamber must be cleaned periodically to remove all such deposits and residue therefrom. Removal of silicon dioxide deposits is accomplished by introducing a fluoride-containing etchant gas such as NF3 (for example) into the chamber and striking a plasma to perform a chamber cleaning operation.
One limitation of inductively coupled plasma reactors is that they require more time to perform a chamber cleaning operation due to their larger volumes, low operational pressure, non-conductive ceilings and lack of bias of the chamber wall surfaces. Certain residues, such as silicon dioxide, typically have relatively low etch rates at low ion energies, and therefore can be difficult to remove by inductively coupled plasmas.
One problem with such a chamber cleaning operation is that the various surfaces of the reactor chamber interior are not cleaned (etched) at the same rate, due to differences in location in the chamber and differences in materials. The plasma density and ion energies are not the same at all locations in the chamber, so that differences in cleaning (etch) rates are quite typical. Moreover, during wafer processing (such as CVD processing), deposits build up more thickly on certain chamber surfaces than on others, so that the deposit or residue thickness is non-uniform throughout the chamber interior. For example, in the etch reactor of the Collins et al. European application referenced above, the etch-process residue is much thicker in the center of the ceiling than at the edge of the ceiling, due to plasma density non-uniformity.
As a result, some metal surfaces are thoroughly cleansed of residue or deposits before others, and the cleaned metal surfaces (e.g., aluminum) are etched while the remaining contaminants are removed from the other still-unclean surfaces. For example, in the case of the flat ceiling electrode of the Collins et al. European application, the edge of the ceiling electrode quickly becomes clean while the center portion still has a thick residue coating remaining thereon. The exposed metal surface of the electrode edge shunts RF energy away from the still-unclean center portion, thereby preventing any further cleaning of the center portion. If a fluoride gas such as NF3 is employed to clean the chamber, the metal surfaces react with the etchant gas during the cleaning operation to form various aluminum fluorides, which re-deposit onto various chamber surfaces, including the ones that are still unclean with other residues or deposits. Such aluminum fluorides etch very slowly or are virtually impervious to being etched (particularly in an inductively coupled plasma), thus masking the prior deposits (e.g., silicon dioxide) underneath. This deposition of aluminum fluorides during the cleaning operation prevents the removal of the remaining residues (e.g., silicon dioxide), so that the reactor chamber cannot be thoroughly cleaned, even by extending the duration of the chamber cleaning operation.
A related problem is that some chamber dielectric surfaces (for example, quartz surfaces) are consumed rapidly during a chamber cleaning operation, particularly in an inductively coupled plasma, and must therefore be frequently replaced at great cost, a significant disadvantage.
The invention is embodied in a method of operating a plasma reactor having a vacuum chamber, a wafer-holding pedestal near the bottom of the vacuum chamber, gas injection ports and a vacuum pump, a generally planar disk-shaped conductive ceiling electrode at the top of the chamber, a helical coil antenna adjacent the ceiling electrode and a base generally coinciding with a circumference of the vacuum chamber and a switch for individually connecting each one of the coil antenna, the ceiling electrode and the wafer pedestal to any one of (a) a respective RF power source (b) ground or (c) a floating potential (i.e., unconnected), the method including a chamber cleaning operation of removing any wafer from the wafer-holding pedestal, creating a vacuum in the chamber with the vacuum pump while introducing an etchant gas into the chamber through the gas injection ports, and operating the switch so as to apply a RF energy to the ceiling electrode while initially refraining from applying RF energy to the coil antenna, so as to strike a predominantly capacitively coupled plasma in the vacuum chamber.
The invention is also embodied in a method of operating a plasma reactor having a vacuum chamber, a wafer-holding pedestal near the bottom of the vacuum chamber, gas injection ports and a vacuum pump, a generally planar disk-shaped conductive ceiling electrode at the top of the chamber, a helical coil antenna adjacent the ceiling electrode and a base generally coinciding with a circumference of the vacuum chamber and a switch for individually connecting each one of the coil antenna, the ceiling electrode and the wafer pedestal to any one of (a) a respective RF power source, (b) ground or (c) a floating potential, the method including, whenever the reactor is to be operated in an inductive coupling mode, operating the switch so as to apply RF power to the coil antenna while grounding the ceiling electrode, and whenever the reactor is to be operated in a capacitive coupling mode, operating the switch so as to apply RF power to the ceiling electrode, and whenever the reactor is to be cleaned, cleaning the reactor by applying RF power to the ceiling electrode and to the coil antenna while introducing an etchant gas into the vacuum chamber.
The invention is further embodied in a method of operating a plasma chemical vapor deposition reactor having a vacuum chamber, a wafer-holding pedestal near the bottom of the vacuum chamber, gas injection ports and a vacuum pump, a generally planar disk-shaped conductive ceiling electrode at the top of the chamber, a helical coil antenna adjacent the ceiling electrode and a base generally coinciding with a circumference of the vacuum chamber and a switch for individually connecting each one of the coil antenna, the ceiling electrode and the wafer pedestal to any one of (a) a respective RF power source, (b) ground or (c) a floating potential, the method including performing a chemical vapor deposition on a wafer in the pedestal by introducing a deposition precursor gas into the chamber while maintaining an inductively coupled plasma therein by operating the switch so as to apply RF power to the coil antenna while grounding the ceiling electrode, and cleaning the reactor by introducing a precursor cleaning gas into the chamber while maintaining a capacitively coupled plasma in the chamber by operating the switch so as to apply RF power to the ceiling electrode.